1. Field of the Invention
Embodiments of the invention relates to an apparatus and method for manufacturing a light-emitting diode, and more particularly to an apparatus and method for manufacturing a light-emitting diode, which is capable of separating a semiconductor layer from a substrate to manufacture a vertical type light-emitting diode.
2. Discussion of the Related Art
A light-emitting diode (hereinafter, referred to as “LED”) is a well-known semiconductor device for converting electrical current into light. The LED emits light when electrons in an active layer made of a semiconductor material are excited from a valence band to a conduction band across a band gap so as to transit the valence band. This transition of the electrons enables the emission of light depending on the band gap energy. Thus, wavelength or color of the light emitted by the LED is determined based on the type of semiconductor material in the active layer since the band gap is one of the specific characteristics of different types of semiconductor materials.
The LED is used for emitting light in various colors, such as red, green, blue, and yellow. However, the LED has a limitation in that it is a monochromatic light source. There may be a requirement for the emission of white light, which includes all of the red, green, and blue lights. For example, a notebook computer using a liquid crystal display (hereinafter, referred to as “LCD”) uses a backlight unit emitting white light. Typically, the white light is provided by an incandescent bulb or a fluorescent lamp. In the case of the incandescent bulb, it has the advantage of being inexpensive but the disadvantages of a very short lifetime and a low light-emitting efficiency. The fluorescent lamp has a higher light-emitting efficiency than the incandescent bulb but still has the disadvantage of a short lifetime. Further, the fluorescent lamp has the additional disadvantages of being relatively large, heavy, and requiring additional expensive electrical components, such as a stabilizer.
A white LED light source may be manufactured by closely positioning red, green, and blue LEDs, which each respectively emit light at an appropriate ratio. In the alternative, a phosphor covered blue LED can emit light that appears white. However, a process for manufacturing the blue LED is not easy since it is difficult to make good-quality semiconductor crystal with the appropriate band gap. Particularly, if using a compound semiconductor of indium phosphide (InP), gallium arsenide (GaAs), and gallium phosphide (GaP), it is difficult to realize a good quality blue LED. In spite of these difficulties, the GaN-based blue LED has been used commercially. A rapid development for technology of the GaN-based blue LED since 1994 has enabled the GaN-based blue LED to surpass the incandescent bulb or fluorescent lamp in terms of efficiency in the field of illumination.
In the case of the InP-based, GaAs-based, and GaP-based LEDs, the semiconductor layer is grown on a conductive substrate such that it is not difficult to manufacture a vertical type LED having a p-n junction structure with first and second electrodes on the top and bottom surfaces, respectively. However, the GaN-based LED uses an insulating substrate made of sapphire (Al2O3) so as to reduce a crystal defect that might occur during the epitaxial growth of GaN on the sapphire substrate. In this case, a horizontal type structure having both first and second electrodes formed on a top surface of the epitaxial GaN layer has been generally adopted since the sapphire substrate is non-conductive.
FIG. 1 is a cross-sectional view of a related art horizontal type LED using a sapphire substrate. As shown in FIG. 1, which is a cross section view illustrating a related art LED 10, an n-GaN layer 12, an active layer 13 having multiple quantum wells, a p-GaN layer 14, and a transparent conductive layer 15 are formed sequentially on a sapphire substrate 11. A first electrode 16 is subsequently formed on a predetermined portion of the transparent conductive layer 15.
Photoresist patterns (not shown) are then formed on the transparent conductive layer 15 including the first electrode 16 by photolithography, wherein the photoresist patterns (not shown) are provided to expose predetermined portions of the transparent conductive layer 15 on which the first electrode 16 is not formed. The transparent conductive layer 15, the p-GaN layer 14, and the active layer 13 are selectively etched under such circumstance that the photoresist patterns are used as a mask. At this time, a portion of the n-GaN layer 12 is etched slightly. A wet etch is preferred to dry etch since GaN layer is difficult to etch. After removing the photoresist patterns by a stripping process, a second electrode 17 is formed on the exposed n-GaN layer 12.
FIG. 2 is a plan view of a related art horizontal type LED using a sapphire substrate. As shown in FIG. 2, in the case of the horizontal type structure, both the first and second electrodes 16 and 17 are on top surfaces. Accordingly, a chip size of the LED 10 should be large enough to ensure sufficient area for contacting the electrodes. The area for contacting electrodes acts as an obstacle to improvement in the output per unit area of a wafer. In addition, manufacturing cost is increased due to the complexity of the wire bonding to both the first and second electrodes 16 and 17 during the packaging process.
The use of a non-conductive sapphire substrate 11 makes it difficult to protect against externally-provided static electricity, thereby increasing failure possibility and lowering device reliability. Also, since the sapphire substrate 11 has low thermal conductivity, it is difficult to transfer heat generated by operation of the LED 10 to the external environment. Because of the low heat transfer capability in the non-conductive sapphire substrate 11, the amount of electric current that can be provided to the LED 10 is limited and thus the output power of the LED 10 is limited. To overcome the problems of the horizontal type LED 10 using the sapphire substrate 11, a vertical type LED, especially a vertical type LED in which the final product does not have a sapphire substrate, has been studied and researched actively.
FIGS. 3 to 7 illustrate sequential steps for manufacturing a vertical type LED. As shown in FIG. 3, serial GaN-based layers 30, including a GaN buffer layer 31, an n-GaN layer 32, a InGaN/GaN/AlGaInN active layer 33 having a multiple quantum well, and a p-GaN layer 34 are formed sequentially on a sapphire substrate 20 by a semiconductor deposition process, such as MOCVD (Metal Oxide Chemical Vapor Deposition) or MBE (Molecular Beam Epitaxy). When a thin film of GaN is grown directly on a sapphire (Al2O3) (001) substrate, a surface uniformity of the thin film might be adversely affected due to a lattice incoherency at the interface between the GaN and the sapphire substrate. In this respect, it is desirable to first form the GaN buffer layer 31 on the sapphire substrate 20, and then to form the serial GaN-based layers 30 on the GaN buffer layer 31. Typically, the sapphire substrate 20 has a thickness of about 330 to 430 μm, and an entire thickness of the serial GaN-based layers 30 is less than about 10 μm.
Then, as shown in FIG. 4, a plurality of trenches 30b are formed through the serial GaN-based layers 30 by an ICP RIE (Inductively Coupled Plasma Reactive Ion Etching) method. The trenches 30b define the individual LEDs. The trenches 30b make the individual LEDs have a shape of a square that is about 200 μm long and 200 μm wide. A trench 30b in and of itself has a width less than about 10 μm. Since the hardness of the serial GaN-based layers 30 is good, the trenches 30b are formed by RIE (Reactive Ion Etching), especially ICP RIE. To form the trenches 30b, a photoresist (not shown) is coated on the GaN-based layers 30 by spin coating, and then the photoresist is treated with a selective exposure and development process to form photoresist patterns (not shown). Then, the GaN-based layers 30 are partially etched by the ICP RIE using the photoresist patterns as an etching mask so as to form the plurality of trenches 30b. 
A laser lift-off process is used to separate the sapphire substrate 20 from the GaN-based layers 30a. The laser can cause fractures or cracks in the GaN-based epitaxial layer as a result of stresses concentrated at the edge of the beam spot due to an inconsistent energy density distribution profile across the laser beam spot. A process of forming the trenches 30b prior to the laser lift-off process is known for addressing such cracks or fractures. That is, the stress causing the fractures or cracks of the GaN-based layers 30a is prevented by the trenches 30b when performing a laser lift-off process to separate the sapphire substrate 20 from the GaN-based layers 30a. Thus, it has been widely known that the process of forming the trenches 30b should be performed prior to the laser lift-off process.
After forming the trenches 30b, as shown in FIG. 5, a conductive supporting layer 40 is formed on the GaN-based layers 30a. Then, the sapphire substrate 20 is separated from the GaN-based layers 30a. To separate the sapphire substrate 20 from the GaN-based layers 30, a laser beam passing through a beam homogenizer (not shown) is applied to the GaN-based layers 30a through the sapphire substrate 20 under such circumstances that the sapphire substrate 20 and the conductive supporting layers 40 are pulled in the opposite directions through the use of vacuum chucks (not shown) adhered thereon. Since stresses can occur at the edge of the laser beam spot (A), the edges of the laser beam spot (A) is positioned to be in the trenches 30b. The alignment of the laser beam edges in the trenches has difficulties in that precise adjustments are required in the timing of the laser beam pulse along with the movement of a stage on which a wafer is loaded thereon.
After the laser beam is sequentially applied to an entire area of an interface between the sapphire substrate 20 and the GaN-based layers 30a through the sapphire substrate 20, the sapphire substrate 20 is separated from the GaN-based epitaxial layer 30a. In this case, the remaining epitaxial layer 30a includes the GaN buffer layer 31, which was in contact with the sapphire substrate 20. Thus, it is necessary to additionally perform a process to remove the GaN buffer layer 31.
As shown in FIG. 6, after removing the GaN buffer layer 31, a contact layer 50 is formed on the respective n-GaN layers 32a. After forming the contact layer 50, the individual LEDs are divided by a dicing process. The dicing process may be performed by various mechanical or chemical methods. FIG. 7 illustrates a cross section view illustrating the final product divided into individual LEDs.